Make the Dough:
In a medium bowl, combine the masarepa, salt, crumbled Cotija cheese, softened butter, and warm water. Knead the mixture until a smooth dough forms. The dough should have the consistency of soft play-dough and should not be sticky. Cover the bowl with a kitchen towel and let the dough rest for 10 minutes.
Check Dough Consistency:
Flatten a small amount of dough between your palms. If the edges crack, the dough is too dry. Add additional water, one tablespoon at a time, until the dough becomes pliable.
Form the Arepas:
With lightly wet hands, divide the dough into 8 equal pieces. Roll each piece into a ball, then flatten it into a circle about 5 inches in diameter and ¼ inch thick.
Stuff with Cheese:
For each arepa, place a few tablespoons of shredded mozzarella cheese in the center of one circle, leaving a ½-inch border. Place another dough circle on top, pressing and sealing the edges to form a disk about ½ inch thick. Repeat with the remaining dough and cheese.
Cook the Arepas:
On the Stovetop: Heat a large skillet over medium-low heat. Add oil or butter to coat the skillet.
On an Electric Griddle: Preheat the griddle to 325ºF. Spread butter on the surface to coat.
Golden Perfection:
Place the arepas on the skillet or griddle. Cook, moving them around occasionally, for about 4-5 minutes or until golden brown. Flip and cook for another 4-5 minutes until the other side is crispy and golden.
Serve:
Remove the arepas from the skillet and serve warm. For extra richness, you can add a pat of butter on top (optional).